The present invention relates to a method of manufacturing an ultrasonic probe and, more particularly, to a method of manufacturing an array ultrasonic probe used in a medical diagnostic apparatus.
In the fields of medical diagnostic apparatuses for examining body cavities and nondestructive inspection apparatuses for probing the interiors of metal welded portions, ultrasonic imaging apparatuses have been used. In such an apparatus, an ultrasonic probe transmits and receives an ultrasonic wave to image the internal state of an object to be examined. The ultrasonic probe of an apparatus of this type uses an ultrasonic transducer made of a piezoelectric ceramic.
Lead zirconium titanate (PZT) has conventionally been used as an ultrasonic probe piezoelectric ceramic. The PZT characteristics such as an electromechanical coupling factor have improved little for the past 20 years. Therefore, a new material has been sought for.
In recent years, a piezoelectric single crystal as a solid solution of lead titanate (PT) and various kind of complex perovskite compound (to be generally called a relaxor) has received a great deal of attention because it has a large electromechanical coupling factor. Known examples of the relaxor are lead-magnesium niobate (PMN) Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3, Pb(In.sub.1/2 Nb.sub.1/2)O.sub.3, etc.
The piezoelectric single crystal consisting of a complex perovskite compound containing PT and a relaxor is generally represented as: EQU Pb[(B1B2).sub.1-x Ti.sub.x ]O.sub.3
wherein B1 is at least one element selected from the group consisting of Mg, Sc, Ni, In, and Yb, and B2 is at least one element selected from the group consisting of Nb and Ta. This piezoelectric single crystal material contains 0 to 55 mol % of lead titanate. That is, 0&lt;x.ltoreq.0.55.
Such a piezoelectric single crystal allows use of a thin transducer even in low-frequency conditions and has a high sensitivity. The thin transducer requires only a small cutting depth for the diamond wheel blade of a dicing machine in obtaining sliver transducers. Even a thin blade can cut the piezoelectric single crystal vertically to improve the yield and provide an ultrasonic probe having a reduced side lobe. Such a piezoelectric single crystal has a relative dielectric constant equal to or higher than that of a conventional PZT piezoelectric ceramic and is thus excellent in matching with a transmitter/receiver. A high-sensitivity signal, in which the loss by the capacitances of a cable and apparatus is small, can be obtained. The acoustic impedance of such a single crystal is as low as about 65% of ceramics and near to the human body, thus facilitating acoustic impedance matching.
Due to the above advantages, an ultrasonic probe using an ultrasonic transducer made of the above piezoelectric single crystal has a higher signal sensitivity by about 5 dB or more than an ultrasonic probe using the conventional PZT piezoelectric ceramic. Human tomographic images (B mode images) obtained with this ultrasonic probe allow the operator to clearly observe a small change to a morbid state or a deep human tissue.
When an ultrasonic probe using an ultrasonic transducer made of the above piezoelectric single crystal is applied to color flow mapping (CFM) for performing two-dimensional color display of an ultrasonic Doppler shift by a blood flow, a large signal can be obtained from an echo reflected by a small blood cell several .mu.m in diameter.
The piezoelectric single crysta l represented by Pb[(B1B2).sub.1-x Ti.sub.x ]O.sub.3 described above is not polarized in a specific direction after crystal growth. After electrodes are formed on both surfaces of the single crystal, it must undergo poling by applying a voltage to the electrode at a high temperature. Conventionally, poling was performed in an electric field of 1 to 3 kV/mm at a high temperature of about 200.degree. C.
A cardiac probe transducer for an ultrasonic diagnostic apparatus has a standard size of about 15 mm.times.25 mm and an area of more than 2.0 cm.sup.2. When a thin single-crystal transducer having a large area undergoes poling under the above conditions, a large warpage of 1 mm or more may occur in the transducer. When the warped transducer is diced after an acoustic matching layer and backing material are adhered to the upper and lower surfaces of the transducer, cracking readily occurs in the transducer, and the production yield greatly decreases. When an array transducer is formed at a dicing pitch of 200 .mu.m or less, the electrical properties of the respective transducers greatly vary.